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They're not especially rare at these latitudes (I live in southern Finland, ~61°N). You see them fairly frequently on summer nights, rocket launches or not. But what is interesting that nobody seems to have written anything about them until the 20th century, which could indicate that suitable condensation nuclei did not exist in the upper atmosphere before humans started emitting them.

I have the same feeling that these are not very uncommon. My guess is, as I'm also from Sweden, is that the period of time the sun is in the right position for this is much longer at these latitudes.

I'm certain Iv'e seen clouds like these many times, at least 5 times if not more.

> I have the same feeling that these are not very uncommon

The article explicitly states the rarity is at the given latitude:

“Such clouds are very rare at this latitude and also in winter…”

Naturally they chose to over-emphasize the rarity aspect in the headline, otherwise no-one would bother clicking.

Considering how common they're in southern Finland, I'd say you'd only need to get a bit north of Stockholm/Göteborg to see them fairly regularily.